1. Field of the Invention
The present invention relates to a digital transmission system. More particularly, it relates to a digital transmission system employed, for example, in an integrated service digital network (ISDN) for transmitting data via a single transmission line using a full duplex communication mode.
2. Description of the Related Art
In a known digital transmission system operated using the full duplex communication mode, a highly accurate timing recovery operation must be achieved to maintain good quality transmission data, and echo must be completely suppressed for the same reason. The above-mentioned second requirement is especially pertinent to a digital transmission system operated under an echo suppression method (or echo cancellation method), but is not so important in a digital transmission system operated under a time compression method, i.e., the so-called ping-pong method. This is because, in the ping-pong method, a signal sent from a transmitter unit in a transmitting-receiving unit cannot inherently leak into a receiver unit in the same unit, as it can in the echo suppression method.
Namely, in the above digital transmission system, interference in the timing recovery operation is usually caused by an echo signal originating from a signal output by that system, and interference in the echo suppression operation is usually caused by a signal received from a remote transmitting-receiving unit.
In the prior art transmitting-receiving unit, a method for maintaining the high quality of transmission data is directed only to a reduction of the echo signal to a level as low as possible, to attain a more accurate timing recovery. For example, the related echo suppression circuit (often called an echo cancellation circuit, but hereinafter referred to as an echo suppression circuit) works using an that drives the echo signal to a level of almost zero at each sampling point. In this case, even though at each sample point echo signal level is reduced to nearly zero, the remaining non sampled portions of the echo signal, which have not been sampled, are left as they are, i.e., at a relatively high level. Thus, non-sampled portions of the echo signal often reach a level as high as the levels of the received signals. Under such circumstances, it is very difficult for the related timing recovery circuit to achieve a really accurate timing recovery operation, since the received signals used for that operation contain undesired non-sampled echo signal components.
Consequently, the problems occurring in the prior art are, first, a large number of sampling points must be used, and second, the timing recovery circuit must be operated in accordance with a complicated algorithm. The above mentioned first and second methods are not suitable for practical and commercial use from an economical viewpoint.